Tests: Fix RunCMake.GenerateExportHeader use of size_t
Since !5969 (merged) the test uses std::size_t without
including <cstddef>. Also, the type does not exist under std::
on VS 6. Use just size_t instead.
Since !5969 (merged) the test uses std::size_t without
including <cstddef>. Also, the type does not exist under std::
on VS 6. Use just size_t instead.